This invention relates to laser light modulation. In particular, this invention relates to devices that modulate light emitted from a laser.
Two technologies now in use to modulate laser light are acoustic optic modulators (AOM's) and mechanical deflection modulators (MDM's). MDM's are limited in frequency to the kilohertz range. AOM's, while usable at higher frequencies, are very expensiv (costing between $4,000 and $50,000 each).
Another method of modulating laser light employs the use of PLZT. PLZT is lanthanum modified lead zirconate titanate. PLZT is a polycrystalline ceramic which, when subjected to an electric field, rotates the polarization angle of a light wave and, when used with external polarizer, can be used as shutter for modulating laser light. By applying interdigitated electrodes onto a piece of PLZT, an electric field can be induced in the PLZT that modulates light passing through it. If polarized light passes through a PLZT wafer at an angle of 45 degrees with respect to an electric field applied to the PLZT, the plane of polarization is rotated (the amount of rotation as compared to the initial polarization plane being proportional to the strength of the applied field). With no field applied, light passes through the PLZT unaltered.
Rotation of the plane of polarization of light waves passing through PLZT is accomplished by having a light incident upon the PLZT wafer split into two rays that travel at different velocities through the wafer. As the two rays travel through the energized PLZT, one component, which is perpendicular to the electric field, is subjected to a higher index of refraction within the PLZT. When the two rays recombine upon exiting the PLZT, their recombination effectuates a phase shift of the light.
The amount of rotation of the plane of polarization through a PLZT wafer is dependent upon the strength of the electric field impressed upon it. A voltage required to generate a 90 degree rotation in a PLZT wafer is termed the "half wave voltage". Voltages above or below this value will cause more or less rotation.
A problem with prior art PLZT light modulators using interdigitated electrodes is the diffraction of a collimated light beam, including a laser beam, incident upon the surface of the PLZT by the edges of the electrodes. Another problem has been power dissipation by these device when operated at high frequencies. The power dissipation required of a PLZT light modulator is a function of its frequency. A PLZT light modulator operating at 1 MHz may be required to dissipate up to 250 watts of power as heat. Prior art PLZT laser light modulators are capable of dissipating heat through convection only. This convection cooling severely limits the operating frequency and increases the likelihood of a failure of a device. A PLZT laser light modulator which is improved heat dissipation would be an improvement over the prior art.